1. Field of the Invention
The present invention relates to a resonator and a light emitting device, and more particularly, to a high-performance resonator in which point defects are provided in a three-dimensional photonic crystal.
2. Related Background Art
The concept of controlling, for example, transmission and reflection characteristics of an electromagnetic wave by a structure having a size equal to or smaller than a wavelength thereof has been proposed by Yablonovitch (Physical Review Letters, Vol. 58, pp. 2059, 1987). When structures, each of which is equal to or smaller than the wavelength, are periodically arranged, for example, the transmission and reflection characteristics of the electromagnetic wave can be controlled. When the wavelength of the electromagnetic wave is reduced to a wavelength order of light, transmission and reflection characteristics of the light can be controlled by the structures. The construction of the structures is known as a photonic crystal. It is suggested that a reflecting mirror in which there is no light loss and thus reflectance thereof is 100% can be realized in a wavelength region when the structures are used. Therefore, it is said that the concept that the reflectance can be increased to 100% in the wavelength region corresponds to a photonic band gap in contrast with an energy band gap of a conventional semiconductor. A three-dimensional minute periodical structure can provide a photonic band gap for light incident from any direction. Hereinafter, this is referred to as a complete photonic band gap.
When the complete photonic band gap is used, an optical device having a novel function can be provided. For example, an optical device in which point or linear defects are provided in the photonic crystal can be operated as a resonator or a waveguide. According to U.S. Pat. No. 5,784,400, in particular, when a active medium including point defects is excited by an exciting means, a high-efficiency laser device in which light is confined to a very small region to suppress spontaneous emission can be realized. In addition, when a shape of the point defects is controlled, a light emitting pattern can be controlled with high precision.
FIGS. 18A, 18B, 18C, 18D, 18E and 18F show structures of a three-dimensional photonic crystal capable of realizing the complete photonic band gap.
In a resonator produced based on the complete photonic band gap (PBG) realized by the three-dimensional photonic crystal, there are generally a plurality of resonance modes. In a resonator used for the light emitting device or the like, it is necessary to have a desirable confining effect and satisfy a resonance condition at a desirable wavelength. It is necessary to increase an interval between a desirable resonance wavelength and the close resonance wavelength of the resonance mode so that the influence of a resonance mode with a close resonance wavelength, such as mode hopping caused in the case where the complete photonic band gap is applied to a laser resonator, can be avoided.
Japanese Patent Application Laid-Open No. 2004-006567, in which a resonator in which point defects are provided in an inner portion of a woodpile structure shown in FIG. 18B is described, discloses that the resonance wavelength can be controlled by controlling a shape or a position of the point defects having a thickness equal to that of a columnar structure. That is, a ratio between a length in x-axis direction and a length in y-axis direction of the point defect having a rectangular parallelepiped shape is changed or the position of the point defect is shifted in a longitudinal direction of a columnar structure including the point defect, thereby controlling the resonance wavelength and the like. However, when the position of the point defect is shifted, an electric field distribution in the inner portion of the resonator becomes asymmetrical because of the asymmetry of the structure. In particular, when such a resonator is applied to a light emitting device such as a laser, a large unbalance in orientation characteristic of emitted light occurs because of the asymmetry. A maximum shift amount of the point defect is half an arrangement period of columnar structures. Therefore, even when the length ratio of the point defect changes, a range of choice of the resonance wavelength is narrow, so a desirable resonance wavelength is not obtained in some cases. Thus, it is necessary that the resonator using the three-dimensional photonic crystal have a desirable electric field intensity distribution at the desirable resonance wavelength.